Fair Value of Financial Instruments - Assumptions - Convertible Note Hedges (Details)		Jun. 30, 2018 Y $ / shares item	Dec. 31, 2017 Y $ / shares item
Fair Value of Financial Instruments
Derivative asset, valuation technique		us-gaap:ValuationTechniqueOptionPricingModelMember	us-gaap:ValuationTechniqueOptionPricingModelMember
Measurement Input, Risk Free Interest Rate
Fair Value of Financial Instruments
Derivative asset, measurement input	[1]	0.027	0.021
Measurement Input, Expected Term
Fair Value of Financial Instruments
Derivative asset, measurement input | Y		4.0	4.5
Measurement Input, Share Price
Fair Value of Financial Instruments
Derivative asset, measurement input | $ / shares	[2]	19.12	14.99
Measurement Input, Exercise Price
Fair Value of Financial Instruments
Derivative asset, measurement input | $ / shares	[3]	16.58	16.58
Measurement Input, Price Volatility
Fair Value of Financial Instruments
Derivative asset, measurement input	[4]	0.423	0.441
Measurement Input, Expected Dividend Rate
Fair Value of Financial Instruments
Derivative asset, measurement input		0	0

[1] Based on U.S. Treasury yield curve, with terms commensurate with the terms of the Convertible Note Hedges and the Note Hedge Warrants.

[2] The closing price of the Company’s Class A common stock on the last trading day of the quarter ended June 30, 2018 and December 31, 2017, respectively.

[3] As per the respective agreements for the Convertible Note Hedges and Note Hedge Warrants.

[4] Selected volatility based on historical volatility of the Company’s Class A common stock.